This invention relates to a method of controlled cooling for steel strip at high temperatures. More particularly, it relates to a method of controlling the cooling of steel strip to an aimed-for temperature at a desired cooling rate.
The main object of the controlled cooling of steel strip has been to cool it to the aimed-for temperature. This object has been achieved by several methods such as adjusting the number of coolant ejecting nozzles and regulating the quantity of coolant ejected through nozzles. The same holds true with the controlled cooling implemented in the continuous annealing of steel strip, in which, however, the cooling rate also constitutes an important factor. If the primary cooling rate in a continuous annealing process is too low, the degree of supersaturation of the solid solution of carbon in steel drops, as a consequence of which the force to cause the precipitation of carbide lessens and the overaging time lengthens. If, on the other hand, the cooling rate is so high as not to permit end-point control, the strip once cooled to room temperature is reheated to the overaging temperature, with a resulting transgranular fine dispersion of carbide precipitates deteriorating the ductility of the steel.
There arises problems in the manufacture of high-tensile steel plates (such as of the dual-phase structure type), as well. If the cooling rate is too low, much alloying element material will be needed to obtain the desired strength. Too high a cooling rate, on the other hand, fails to provide adequate ductility. Consequently, quenched solid solution of carbon has to be reheated for overaging precipitation at such a low temperature at which the formed martensite does not break. Even this corrective step cannot fully make up for the deterioration in ductility caused by the fine carbide. In other words, the cooling rate should neither exceed nor fall short of the appropriate level. Incidentally, the aimed-for cooling temperature governs the rate at which solid solution of carbon precipitates.
As is now obvious, the cooling rate is an important factor, but there has been no appropriate measures to control it, with the conventional techniques confined to the control of the desired cooling temperature.